Generally, a problem associated with signal receivers having one or more loudspeakers connected to direct-coupled audio power amplifiers is transistor failure in the amplifier. When a transistor in the amplifier fails, it is not uncommon for several amperes of DC current to appear at the loudspeaker. Such excessive current may be of an amount sufficient to burn out or cause a fire at the loudspeaker. Moreover, such currents usually have a deleterious effect on the ammplifier and may burn out several elements therein or even initiate a fire in the amplifier.
One of the common approaches to a solution of the above-described problem is the inclusion of circuit breakers in series with the leads of the loudspeaker. However, circuit breakers in series with the leads of the loudspeaker must be capable of carrying the AC current supplied to the loudspeakers and such currents can reach a figure of 2.5 to 4.5 amperes. Unfortunately, a DC current, derived from a fault condition, and of such an amount is more than enough to burn out any of the commonly employed loudspeakers.
Another known approach to the problem provides fuses in series with the speaker leads or in the B+ or power supply circuitry coupled to the power amplifiers. Unfortunately, fuses are relatively slow acting, suffer from the same limitation as circuit breakers, and are somewhat unpredictable in response. Moreover, fuses are unacceptable because they would not normally be considered a customer replaceable item.
Still another approach to the problem included a triac crowbar circuit in the loudspeaker leads. Also, circuitry with a plurality of transistors and a relay have been utilized to protect the loudspeaker system. However, the above-mentioned circuitry has been found to be relatively expensive of components and installation labor. Moreover, increased components tend to increase the complexity and reduce the reliability of most electrical circuitry.